Force measurement of bimetallic thermal disc

ABSTRACT

An apparatus and method for determining the actuation energy generated by a bimetallic actuator during transit between first and second states of stability. The apparatus and method further determining the threshold or set-point temperature of the bimetallic actuator during transit between bi-stable states. Accordingly, the apparatus and method directly measure both the snap force F and the set-point temperature of the bimetallic actuator during transit.

FIELD OF THE INVENTION

The present invention relates generally to methods for manufacturingthermally responsive bimetallic members, and in particular to methodsfor determining the snap energy generated by snap-action bimetallicmembers during transit between first and second states of stability.

BACKGROUND OF THE INVENTION

Thermally responsive bimetallic members that exhibit a snap-actionresponse are commonly utilized to actuate overheat protection andthermostatic switching mechanisms. One type of such mechanisms is athermostatic switch that utilizes an actuator formed of a bimetallicmaterial having materials of relatively low and high thermal expansioncoefficients joined together along a common interface. Snap-actionbimetallic switching mechanisms typically exhibit two states ofstability with each of these states being responsive to a predeterminedthreshold or set-point temperature. When the switching mechanism sensesa temperature that is below a first lower of these predeterminedset-point temperatures, the thermally responsive member is in one of thetwo stable states. Accordingly, when the sensed temperature is above asecond higher predetermined set-point temperature, the thermallyresponsive member snaps to a second of the two stable states and remainsin this second state while the sensed temperature remains at or abovethis second higher set-point temperature. Should the sensed temperaturebe reduced to the first lower temperature, the temperature of the memberis lowered correspondingly. As a result, the thermally responsive membersnaps back to the first lower temperature state. The difference betweenthe two predetermined set-point temperatures corresponding to therespective first and second states of stability is known as the“differential temperature” of the thermally responsive member.

A known method of manufacturing thermally responsive snap-actionswitches of the variety described above has included a forming operationin which a pre-sized blank of the thermally responsive bimetallic memberis positioned between two opposingly positioned shaping or die members.The shaping members are actuated to engage the bimetallic member,thereby providing the bimetallic member with the desired configurationneeded to achieve snap-action at each of the two predetermined set-pointtemperatures. Such a configuration usually consists of a knee and/orcorresponding bowed portion, a dimpled portion or portions, or a seriesof ridges. Examples of such of formations are described in U.S. Pat. No.3,748,888 and U.S. Pat. No. 3,933,022, each of which is incorporatedherein by reference in its entirety, wherein a thermally responsivesnap-action bimetallic disc is provided.

U.S. Pat. No. 3,748,888 also describes a smoothly formed prior artdisc-shaped snap-action bimetallic member, as illustrated in side viewin FIG. 1. A bimetallic member 1 is formed using a disc of materialformed of two materials 2, 3 having different thermal expansioncoefficients joined together along contiguous surfaces. One of themembers 2 is formed of a material having a relatively high coefficientor rate of thermal expansion, while the other member 3 is formed of amaterial having a low coefficient or rate of thermal expansion relativeto that of the first member 2. The difference in thermal expansioncoefficients between the two members 2, 3 is a factor in determining theset-point temperature at which the resulting bimetallic disc actuator 1operates and in an actuation force F produced by the snap-action. Thedisc-shaped bimetallic member 1 is often circular and, in someinstances, is provided with a small, centrally located aperturetherethrough (not shown). Bimetallic discs of this type are generallyformed by “bumping” a flat circular disc blank with a punch-and-die setto stretch the bimetallic material of the disc into the concavestructure having a depth H1, as illustrated by full line 4 in FIG. 1.The bimetallic disc 1 is formed, for example, with a substantiallyplanar peripheral hoop portion 5 surrounding a central portion 6stretched into a concave configuration. The central portion 6 is mobilerelative to the peripheral hoop portion 5, the central portion 6 movingfrom one side of the peripheral hoop portion 5 to the other as afunction of temperature. The set-point operation temperature and theforce F applied by the snap-action are thus physical characteristics ofthe two members 2, 3 that form the bimetallic member 1.

Generally, when the bimetallic disc 1 is intended to operate at atemperature above ambient temperature, the disc 1 is bumped on the highexpansion rate side 2 to form the central stretched portion 6, wherebythe central portion 6 is stretched to space the inner concave surfacethereof to a depth H1 away from the plane P of the peripheral hoopportion 5, as illustrated by the full line configuration 4. The depth ofpenetration of the punch during the bumping operation determines thedepth H1 and thus is another factor in determining both the upperset-point temperature and the force F applied by the snap-actionoperation of the disc 1. The set-point operation temperature and theforce F applied by the snap-action are thus also structuralcharacteristics of the bimetallic member 1, as also described inabove-incorporated U.S. Pat. No. 3,748,888.

The bimetallic disc 1 is illustrated in FIG. 1 in full line 4 in one ofits two states of stability. Assuming the bimetallic disc 1 is intendedfor operation at a set-point temperature above ambient temperature, thehigh expansion rate side is located on the surface 2 and the lowexpansion rate side is along the surface 3. If the bimetallic disc 1 isintended for operation at a set-point temperature below ambienttemperature, the bimetallic disc 1 is formed in the opposite shape withthe low expansion side located on the surface 2 and the high expansionrate side along the surface 3. For purposes of explanation only, thebimetallic disc 1 shown in FIG. 1 is assumed to be intended foroperation at a set-point temperature above ambient temperature.Accordingly, at a temperature well below the upper set-point temperaturethe bimetallic disc 1 is configured with the central stretched portion 6in an upwardly concave state, as shown by the upper dotted line 7.

As the temperature of the bimetallic disc 1 is raised to approach itsupper set-point operating temperature, the high expansion rate material2 begins to stretch, while the lower expansion rate material 3 remainsrelatively stable. As the high expansion rate material 2 expands orgrows, it is restrained by the relatively more slowly changing lowerexpansion rate material 3. Both the higher and lower expansion ratesides 2, 3 of the bimetallic disc 1 become distorted by the thermallyinduced stresses, and the central mobile portion 6 of the bimetallicdisc 1 changes configuration with a slow movement or “creep” action fromthe upper dotted line configuration 7 to the full line configuration 4.The inner concave surface of the central mobile portion 6 is spaced thedepth H1 away from the plane P of the peripheral hoop portion 5. Thefull line configuration 4 is considered herein to be a first state ofstability.

As soon as the temperature of the bimetallic disc 1 reaches its upperpredetermined set-point temperature of operation, the central stretchedportion 6 of the disc 1 moves with snap-action downward through theunstretched hoop portion 5 to the second state of stability with theinner concave surface of the central mobile portion 6 spaced a distanceH2 away from the plane P of the peripheral hoop portion 5, as shown bythe phantom line 8. If the temperature of the bimetallic disc 1 israised to a still higher temperature, the high expansion rate material 2continues to expand at a greater rate than the relatively lowerexpansion rate material 3 joined thereto. As a result of this continueddifferential expansion, the central mobile portion 6 of the bimetallicdisc 1 continues to creep toward a state of even greater downwardconcavity, as shown by the second lower dotted line configuration 9.

As the temperature of the bimetallic disc member 1 is reduced from thehigh temperature toward the lower predetermined set-point temperature ofoperation, the central mobile portion 6 of the bimetallic disc 1 movesfrom the state of extreme concavity, as shown by the lower dotted line9, toward the second state of stability indicated in phantom 8. As thetemperature of the bimetallic disc 1 is reduced below the second orlower predetermined set-point temperature of operation, the material 2having the relatively larger thermal coefficient also contracts orshrinks more rapidly than the other material 3 having the relativelysmaller thermal coefficient. The bimetallic disc 1 changes configurationwith a similar slow movement or creep action from the state of greatestdownward concavity toward the second state of stability indicated inphantom 8. As the bimetallic disc 1 reaches the lower set-pointtemperature, the central stretched portion 6 snaps back through theunstretched hoop portion to the first state of stability, as shown bythe upper full line 4. If the temperature is decreased still further,the differential expansion between the high and low rate materials 2, 3causes the central mobile portion 6 to continue to creep toward thestate of greatest upward concavity, as shown by the upper dotted line 7.

Many thermal switch designs use one of the bimetallic discs 1 that snapinto a different state of concavity at a predetermined threshold orset-point temperature, thereby closing a contact or other indicator tosignal that the set-point has been reached. The speed at which thebimetallic disc actuator 1 changes state is commonly known as the “snaprate.” As the term implies, the change from one bi-stable state to theother is not normally instantaneous, but is measurable. A slow snap ratemeans that the state change occurs at a low rate of speed, while a fastsnap rate means that the state change occurs at a high rate of speed.Accordingly, in some known configurations of switch and indicatordevices, a slow snap rate results in arcing between the operativeelectrical contacts. Slow snap rates thus limit the current carryingcapacity of the thermal switch or indicator device. In contrast, a fastsnap rate means that the change in state occurs rapidly, which increasesthe amount of current the thermal switch or indicator device can carrywithout arcing. The temperature rate of change affects the snap rate. Aslower temperature rate of change tends to slow the snap rate, while afaster temperature rate of change usually results in a faster snap rate.While some applications provide fast temperature rates, switches andindicators experience very slow temperature rates in many otherapplications. In some applications, the temperature rates may be as lowas about 1 degree F. per minute or less. For long-term reliability thedevice must operate in these very slow temperature application rateswithout arcing.

Furthermore, a minimum force F is required to actuate the contacts. Asdescribed above, the force F is thermally induced in the bimetallic disc1 as the result of both the depth H1 of the concavity formed in the disc1, and the differential thermal expansion rate between the high and lowexpansion rate sides 2, 3 thereof. The force F produced during transitfrom one state of stability to the other state must be sufficientlypowerful to overcome the contact restoring force in order to actuate thedevice. For example, the force F must be sufficient to overcome arestoring spring force in a flexible switch contact. If a bimetallicdisc 1 with insufficient snap force F is installed into a thermal switchor other indicator device, the switch or device may fail prematurely,requiring replacement of the bimetal disc 1 or replacement of the entiremechanism.

Typically, the snap force F generated by the individual bimetallic disc1 is tested prior to installation in the using device. For example, thebimetallic discs 1 are pre-tested under maximum load to ensure that eachexerts sufficient snap force F at temperature application rates of about1 degree F. per minute or less to actuate the device's contact withoutarcing. One known method of ensuring the snap quality of the bimetallicdisc 1 is testing of the force F produced during actuation of the snapin situ. Pre-testing is thus accomplished by placing the disc 1 in theintended device and testing the fully assembled thermal switch or otherindicator mechanism. Pre-testing is thus cumbersome and time consuming.Furthermore, the present in situ testing process is typically a simplego/no-go test in which marginally performing bimetallic discs 1 mayremain undiscovered. The manufacturer may thus be forced to employexcessively conservative quality control measures.

SUMMARY OF THE INVENTION

The present invention is a method and means for determining an amount ofenergy released by a thermally responsive snap-action bimetallicactuator. The method of the invention includes forming a bimetallic dischaving a mobile center portion surrounded by a substantially immobileperipheral portion; qualifying an energy released by transit of themobile portion from a first side of the peripheral portion to a secondopposite side of the peripheral portion during operation of a snapaction; and subsequently assembling the disc into operative relationshipwith a movable indicator portion of a sensing device.

According to another aspect of the invention, the method of theinvention includes presenting a thermally responsive snap-actionbimetallic actuator to a sensing portion of a force sensing device whilethe actuator is configured in a first pre-snap state wherein a mobileportion of the actuator is spaced away from the sensing portion of theforce sensing device, and determining a force generated by the actuatorduring transit to a second post-snap state wherein the mobile portion ofthe actuator is moved into forceful contact with the sensing portion ofthe force sensing device.

According to one aspect of the invention, presenting the actuator to thesensing portion of the force sensing device includes thermallyactivating the actuator to transit to the second post-snap state.

According to another aspect of the invention, presenting the actuator tothe sensing portion of the force sensing device includes placing theactuator on a support structure configured to support the actuator.

According to another aspect of the invention, determining a forcegenerated by the actuator includes detecting a peak force generated bymoving the mobile portion of the actuator into forceful contact with thesensing portion of the force sensing device.

According to another aspect of the invention, presenting the actuator tothe sensing portion of the force sensing device includes positioning theactuator in proximity to a thermal stage, and activating the thermalstage. Activating the thermal stage includes activating the thermalstage in a controlled manner. According to another aspect of theinvention, determining a force generated by the actuator includesdetermining an energy-temperature rate relationship exhibited by theactuator.

According to still another aspect of the invention, the method of theinvention also includes assembling the actuator into operativerelationship with a movable indicator portion of a thermal sensingdevice.

According to other aspects of the invention, the invention provides anenergy measuring device having a means for supporting a bimetallicmember in a first pre-snap state; a means for qualifying an energyreleased by the bimetallic member during transit from the first pre-snapstate to a second post-snap state, the qualifying means being positionedrelative to the supporting means to be engaged by the bimetallic memberin the second post-snap state; and a means for thermally activating thebimetallic member, the thermally activating means being positionedrelative to the supporting means for thermally activating the bimetallicmember to transit from the first pre-snap state to the second post-snapstate.

According to another aspect of the invention, the means for qualifyingthe released energy includes means for measuring a force generated bythe bimetallic member, and may also include means for measuring a peakforce generated by the bimetallic member during the transit from thefirst pre-snap state to the second post-snap state.

According to another aspect of the invention, the thermally activatingmeans of the device includes means for thermally activating thebimetallic member in a controlled manner, including for example, meansfor heating or cooling the bimetallic member at a controlled rate oftemperature change.

According to another aspect of the invention, the means for supportingthe bimetallic member in the first pre-snap state includes meansstructured to support a substantially immobile peripheral portion of thebimetallic member while a substantially mobile portion of the bimetallicmember that is located centrally to the peripheral portion is disengagedfrom the qualifying means.

BRIEF DESCRIPTION OF THE FIGURES

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 illustrates a known bimetallic actuator disc;

FIG. 2 is a top plan view of the thermally responsive device of thepresent invention embodied as a snap-action thermal switch;

FIG. 3 is a cross-sectional view of the snap-action thermal switchillustrated in FIG. 2, wherein the electrical contacts form a closedcircuit;

FIG. 4 is another cross-sectional view of the snap-action thermal switchillustrated in FIG. 2, wherein the electrical contacts form an opencircuit;

FIG. 5 illustrates the thermally responsive bimetallic member realizedby the method of the invention embodied as a bimetallic disc actuator;

FIG. 6 illustrates the testing apparatus of the invention embodied as adisc snap-energy tester;

FIG. 7 illustrates the sizing of an intermediary drive pin positionedbetween a sensitive operational portion of a force indicator of thetesting apparatus of the invention shown in FIG. 6 and an actuatedbimetallic disc that is configured in a second post-snap state; and

FIG. 8 illustrates the disc snap-energy tester of the invention embodiedwithout an intermediary drive pin.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the Figures, like numerals indicate like elements.

The present invention is an apparatus and method for determining thesnap energy or snap force F generated by a bimetallic actuator duringtransit between first and second states of stability. The inventionfurther provides an apparatus and method for determining of thethreshold or set-point temperature of the bimetallic actuator duringtransit between bi-stable states. Accordingly, the apparatus and methodprovide for directly measuring both the snap force F and the set-pointtemperature of the bimetallic actuator during transit.

FIG. 2 is a top plan view and FIGS. 3 and 4 are cross-sectional views ofthe thermally responsive device of the present invention embodied as asnap-action thermal switch 10. The snap-action is driven by a thermallyresponsive snap-action actuator of the present invention embodied as asnap-action bimetallic disc actuator 12, wherein the bimetallic discactuator 12 includes a minimum snap force F generated during transitbetween bi-stable states at a predetermined set-point temperature asdetermined according to the method of the invention. For example, themethod is operated using the apparatus of the invention. The thermalswitch 10 also includes a pair of electrical contacts 14, 16 that arerelatively movable under the control of the disc actuator 12. Theelectrical contacts 14, 16 are mounted on the ends of a pair ofspaced-apart, electrically conductive terminal posts 20, 22 that aremounted in a header 24 such that they are electrically isolated from oneanther. For example, terminal posts 20, 22 are mounted in the metallicheader 24 using a glass or epoxy electrical isolator 26.

As illustrated in FIGS. 3 and 4, the electrical contacts 14, 16 aremoveable relative to one another between an open state (FIG. 4) and aclosed state (FIG. 3). For example, the movable contact 16 is affixed toan electrically conductive carrier 28 that is embodied as an armatureformed of an electrically conductive spring material. The armature 28 isaffixed in turn in a cantilever fashion to the electrically conductiveterminal post 22 such that a spring pressure S of the armature 28operates to bias the movable contact 16 toward the fixed contact 14 tomake electrical contact therewith, as shown in FIG. 3. The electricalcontacts 14, 16 thus provide an electrically conductive path between theterminal posts 20, 22 such that the terminal posts 20, 22 are shortedtogether.

The disc actuator 12 is spaced away from the header 24 by a spacer ring30 interfitted with a peripheral groove 32. A cylindrical case 34 fitsover the spacer ring 30, thereby enclosing the terminal posts 20, 22,the electrical contacts 14, 16, and the disc actuator 12. The case 34includes a base 36 with a pair of annular steps or lands 38 and 40around the interior thereof and spaced above the base 36. The lower edgeof the spacer ring 30 abuts the upper case land 40. A peripheral edgeportion 42 of the disc actuator 12 is captured within an annular groovecreated between the lower end of the spacer ring 30 and the lower caseland 38. The disc actuator 12 operates the armature spring 28 toseparate the contacts 14, 16 through the distal end 44 of anintermediary striker pin 46 fixed to the armature spring 28. Separationof the contacts 14 and 16 creates an open circuit condition.

As shown in FIG. 3, while the thermal switch 10 is maintained below apredetermined set-point temperature, the disc actuator 12 is maintainedin a first state with the bimetallic disc actuator 12 withdrawn into aspace 47 between the lower case land 38 and the case end 36. In thisfirst state, an inner concave surface 48 of an arcuate or dish-shapedcentral mobile portion 49 of the bimetallic disc actuator 12 is spacedaway from the intermediary striker pin 46, whereby the actuator force Fis removed from the armature 28. The relatively moveable electricalcontacts 14, 16 are moved together under the spring pressure S suppliedby the armature 28 and thereby form a closed circuit. The spacingbetween the inner concave surface 48 of the bimetallic disc actuator 12and the distal end 44 of the striker pin 46 is sufficient to preventslight movement of the actuator disc 12 effecting contact engagement.

In FIG. 4, the armature 28 is operated under the control of thebimetallic disc actuator 12, which inverts the central mobile portion 49with a snap-action as a function of a predetermined set-pointtemperature between bi-stable states of opposite concavity. As shown inFIG. 4, in response to an increase in the sensed ambient temperatureabove the predetermined set-point, the central mobile portion 49 invertsin a high speed, forceful snap-action into a loaded relationship withthe electrical contacts 14, 16, whereby the inner concave surface 48 isinverted into an outer convex surface 48 that rapidly engages the distalend 44 of the intermediary striker pin 46. The snap-action of thebimetallic disc actuator 12 operates at the predetermined set-pointtemperature to rapidly generate a force F that is predetermined to besufficient to overcome the spring pressure S of the armature 28.Accordingly, operation of the bimetallic disc actuator 12 flexes themovable contact 16 away from the fixed contact 14. For example, the discactuator 12 operates through the intermediary striker pin 46 fixed tothe armature spring 28 to pivot the armature spring 28 upwardly, therebyseparating the contacts 14, 16. Separation of the contacts 14, 16creates an open circuit condition.

The snap rate and force F with which the central mobile portion 49 ofthe bimetallic disc actuator 12 changes state determine the speed withwhich the contacts 14, 16 are allowed to come together to make thecircuit, or are separated to break the circuit. The make and breakspeeds determine how much current can be carried without undesirablearcing between the contacts 14, 16. Faster make and break speedsincrease the amount of current the thermal switch 10 can carry withoutarcing, and thus increase reliability while extending the useful life ofthe thermal switch 10.

According to one embodiment of the invention, the bimetallic discactuator 12 is fabricated to transit or snap the central mobile portion49 at a high rate while exerting at least a minimum force F.Accordingly, the snap-action of the bimetallic disc actuator 12 changesstate within about 1 millisecond while exerting sufficient force F toovercome the spring pressure S of the armature 28 to break the circuit.The movable contact 16 is thus flexed away from the fixed contact 14rapidly, so that little arcing occurs. The current carrying capacity ofthe thermal switch 10 is thereby maximized.

When the ambient temperature sensed by the bimetallic disc actuator 12is reduced below the predetermined set-point, the fast snap rate rapidlyreturns the central mobile portion 49 to the spaced-away,noninterference relationship with the electrical contacts 14, 16, asshown in FIG. 3. The relatively moveable electrical contacts 14, 16 arerapidly moved together again under the spring pressure S of the armature28. A closed circuit between the two terminal posts 20, 22 is therebyformed. Accordingly, one embodiment of the invention provides asnap-action that changes state of the bimetallic disc actuator 12 withinabout 1 millisecond. The spring pressure S of the armature 28 causes themovable contact to follow the retreating central mobile portion 49 ofthe disc actuator 12. The movable contact 16 is thus flexed into contactwith the fixed contact 14 rapidly, so that arcing is minimized and thecurrent carrying capacity of the thermal switch 10 is maximized.

The thermal switch 10 is sealed to provide protection from physicaldamage. The thermal switch 10 is optionally hermetically sealed with adry Nitrogen gas atmosphere having trace Helium gas to provide leakdetection, thereby providing the contacts 14, 16 with a clean, safeoperating environment.

FIG. 5 illustrates the thermally responsive bimetallic member realizedby the method of the invention embodied as the bimetallic disc 12. Thecentral mobile portion 49 of the bimetallic disc 12 generates apredetermined minimum snap force F at a predetermined set-pointtemperature during temperature application at a predetermined rate asdetermined by a test performed in a prescribed manner according to themethod of the invention. For example, the method is operated using theapparatus of the invention. The bimetallic disc actuator 12 according tothe invention is initially fabricated according to generally knownmethods, as described in connection with FIG. 1. For example, athermally responsive bimetallic material 50, such as ASTM-1, is selectedaccording to known criteria for forming a bimetallic actuator. Suchthermally responsive bimetallic material includes a first metallicmaterial 52 having a first coefficient or rate of thermal expansion anda second metallic material 54 having a second relatively higher rate ofthermal expansion. The first and second metallic materials 52, 54 of thethermally responsive bimetallic material 50 are bonded together alongone contiguous surface 56.

The bimetallic material 50 is formed into a blank of desired shape andsize. For example, a flat, round disk-shaped blank is formed having adiameter D sized to move freely within the annular groove created in thethermal switch assembly 10 between the lower end of the spacer ring 30and the lower case land 38.

The disk-shaped blank is subjected to a forming or “bumping” operationin which the blank of thermally responsive bimetallic material ispositioned between two opposingly positioned shaping members (notshown). The shaping members are actuated to engage the disk-shaped blankof bimetallic material 50, thereby forming the bimetallic disc 12 withthe central mobile portion 49 having a configuration that achievesforceful snap-action at each of the two predetermined set-pointtemperatures. For example, the disk-shaped blank is placed in a femaledie which supports the blank along its peripheral edge portion 42. Amale punch having a spherical end is pressed against the center of thedisc to stretch the metal and form the central mobile portion 49 havingthe inner dish-shaped concave surface 48. The peripheral edge portion 42either retains its substantially planar initial shape, or is formed bythe shaping members with a substantially planar shape. Examples of suchdish-shaped discs are illustrated in U.S. Pat. Nos. 2,717,936 and2,954,447, each of which is incorporated herein by reference in itsentirety. The formed bimetallic disc may be subsequently subjected to aheat treatment operation in order to achieve forceful snap-action of thecentral mobile portion 49 at each of the two predetermined set-pointtemperatures.

The method and apparatus of the present invention reduce or eliminatereiterative processes of screening the snap force F by installing thebimetallic disc actuators 12 into fully assembled thermal switches 10 orother thermally responsive indicator mechanisms. In contrast to currentreiterative screening methods, the method and apparatus of the presentinvention determine the energy, i.e. the snap force F, generated duringthe snap-action transit of the central mobile portion 49 of thebimetallic disc 12 before it is assembled into a sensor mechanism. Lowenergy bimetallic discs 12 are identified and removed from a pool ofusable hardware. The method and apparatus of the present inventionthereby result in predictable product delivery since the measurement ofthe snap force F prevents imbalanced mechanisms from reaching customers.Improved predictability satisfy customer requirements for quality andreliability while reducing manufacturing costs.

FIG. 6 illustrates the testing apparatus of the invention embodied as adisc snap-energy tester 60. The snap energy of the central mobileportion 49 of each individual disc 12 is effectively determined bythermally inducing the snap-action state transformation under controlledconditions and measuring the snap force F prior to installation into asensor mechanism.

According to one embodiment of the invention, the disc energy tester 60is an apparatus having a support structure 62 upon which a measuringdevice 64 is mounted. The measuring device 64 includes a supportingmeans 66 for supporting one of the bimetallic discs 12 in a mannersubstantially similar to the intended application. For example, thesupporting means 66 is embodied as a stand or column formed to resemblea portion of the intended thermal switch 10 or other indicator device,such as the cylindrical case 34 shown in FIGS. 3 and 4. The supportingmeans 66 is, for example, embodied having a first annular step or land68 around the interior thereof and spaced above a base or lower edge 70thereof. The land 68 is sized to support the peripheral edge portion 42of the bimetallic disc 12 above the base 70. A suitable force indicator72, such as a conventional pressure-sensing transducer, is spaced abovethe bimetallic disc 12. For example, the force indicator 72 is suspendedfrom an arm portion 73 of the measuring device 64 overhanging thesupport structure 62 and the supporting means 66. The force indicator 72is of a type that senses a maximum or peak force F_(P) of the snap anddisplays the value of the peak force F_(P) in a useful manner.Optionally, the force indicator 72 is positioned in sufficiently closeproximity to the bimetallic disc 12 that the snap force F is measureddirectly. Alternatively, the force indicator 72 is spaced away from thebimetallic disc 12 and the snap force F is measured through anintermediary member 74.

A means 76 for thermally inducing the snap-action transformation in acontrolled manner is provided. For example, the support structure 62 isembodied as containing a thermal stage 78 capable of inducing apredetermined, adjustable and controllable rate of temperature change inthe bimetallic disc 12 under test. The thermal stage 78 includesadjustable and controllable heating sources. For example, the heatingsources are thermoelectric devices such as commercially availableelectrical resistive devices. For actuation temperatures above ambient,controlled cooling is provided by alternate energizing and de-energizingof the controllable heating sources. Alternatively, cooling sources areprovided as a manifold formed in the support structure 62 and filledwith a coolant such as liquid nitrogen (LN2) or carbon dioxide (CO2)derived from a conventional external source (not shown). Temperaturecontrol is provided external to the energy tester 60, for example, by aconventional programmable controller 80 such as are well-known forproviding precise control of thermal application rate and steady-statetemperatures.

The disc energy tester 60 shown in FIG. 6 is embodied for testing thebimetallic disc 12 intended to actuate the contacts 14, 16 in a thermalswitch 10. Accordingly, the supporting means 66 is embodied as ahollowed column structure formed in a tube or sleeve configurationsimilar to the cylindrical case 34 shown in FIGS. 3 and 4. The materialof the hollowed column supporting means 66 is a thin stainless steel,which is either the same or a similar material used to form thecylindrical case 34. The sizes and relative spacings of the land 68around the interior of the hollowed column supporting means 66 and thebase 70 thereof are similar to the land 38 and the base 36,respectively, of the cylindrical case 34. Furthermore, the supportingmeans 66 embodied as a hollowed column structure includes a secondannular step or land 82 around the interior thereof. The second land 82is paired with the land 68 and spaced above it relative to the base 70.The pair of annular lands 68, 82 are formed to resemble the pair ofannular lands 38, 40 around the interior of the thermal switch case 34.The spacing between the land 82 and each of the respective land 68 andbase 70 is similar to the spacing between land 40 and the respectiveland 38 and base 36 of the case 34. The supporting means 66 is thussubstantially identical to the support structure provided by theintended device.

As embodied in FIG. 6, the force indicator 72 is spaced away from thebimetallic disc 12. The snap force F is applied to the force indicator72 through the intermediary member 74, which is embodied as a stiff,lightweight drive pin formed of titanium or another suitable material.The intermediary drive pin 74 may be hollowed to reduce its weight. Alighter weight drive pin 74 has less effect on the measurement of thesnap force F. A first end portion 84 of the intermediary pin 74 isconfigured similarly to the striker pin 46 and contacts the innerconcave surface 48 of the bimetallic disc 12 when it is configured in afirst pre-snap state, wherein mobile central portion 49 of thebimetallic disc 12 is withdrawn into a space 85 between the firstannular land 68 and the base 70 of the hollowed column structuredsupporting means 66, its inner concave surface 48 being spaced away fromthe force indicator 72. Alternatively, the first portion 84 of theintermediary pin 74 is spaced slightly away from the surface 48 of thebimetallic disc 12 under test, whereby the weight of the intermediarypin 74 is absent from the bimetallic disc 12 in the first pre-snapstate. Accordingly, the first portion 84 of the intermediary drive pin74 is spaced slightly away from the inner concave surface 48 of thebimetallic disc 12. The intermediary pin 74 is thus positioned similarlyto the striker pin 46 of the thermal switch 10 when the central mobileportion 49 of the bimetallic disc 12 is withdrawn into the space 47between the lower case land 38 and the case end 36 so that the contacts14, 16 are configured in a closed circuit condition.

A second end portion 86 of the pin 74 is configured in a shape suitablefor striking the force indicator 72. The pin 74 has a length sized tosubstantially but not completely fill the space between the between thebimetallic disc 12 and the force indicator 72 when the bimetallic disc12 is configured in the first pre-snap state having the inner concavesurface 48 of its central mobile portion 49 spaced away from the forceindicator 72, whereby the snap force F is absent from the forceindicator 72. In other words, the pin 74 is short enough that it doesnot press on the force indicator 72 when the bimetallic disc 12 iswithdrawn into the space 85 between the first annular land 68 and thebase 70 of the hollowed column structured supporting means 66.

FIG. 7 illustrates that the pin 74 is further sized to drive against asensitive operational portion 88 of the force indicator 72 when thebimetallic disc 12 is configured in a second post-snap state, whereinthe inner concave surface 48 of the central mobile portion 49 isinverted into an outer convex surface 48 directed toward the forceindicator 72. Accordingly, the intermediary drive pin 74 completelyfills the space between the bimetallic disc 12 and the force indicator72 and transmits the snap force F of the bimetallic disc 12 to thesensitive operational portion 88 of the force indicator 72 for themeasurement thereof. Stated differently, during testing the centralmobile portion 49 of the bimetallic disc 12 is inverted into an outerconvex surface 48 that rapidly engages the first end 84 of theintermediary drive pin 74. The drive pin 74 transmits the energy in thesnap of the bimetallic disc 12 through the second end 86 of the pin 74into the sensitive portion 88 of the force indicator 72.

In summary, the support means 66 provides the annular land 68 to supportthe bimetallic disc along its peripheral edge 42 and within the space 85between the annular land 68 and the base 70 of the hollowed columnstructure. In its first pre-snap state the central mobile portion 49 ofthe bimetallic disc 12 is withdrawn into the space 85 so that its innerconcave surface 48 is spaced away from the force indicator 72. The snapforce F is thus absent from the force indicator 72. This first pre-snapstate is consistent with the closed circuit condition of the thermalswitch 10 wherein the inner concave surface 48 of the central mobileportion 49 is spaced away from the intermediary striker pin 46 and theactuator force F is removed from the armature 28, which permits thecontacts 14, 16 to close. During testing the bimetallic disc 12 isinverted into its second post-snap state wherein the inner concavesurface 48 of the central mobile portion 49 is inverted into an outerconvex surface 48 that rapidly engages the pin 74 and presses it intothe force indicator 72. This second post-snap state is consistent withthe open circuit condition of the thermal switch 10 wherein the outerconvex surface 48 rapidly engages the intermediary striker pin 46 andthe actuator force F is applied to the armature 28, which forces thecontacts 14, 16 open. The apparatus of the invention thus tests the snapforce F applied by the central mobile portion 49 of the bimetallic disc12 under test during transit from the first pre-snap state to the secondpost-snap state.

According to one embodiment of the invention, a cylindrical spacer 90 ismounted on the second land 82 formed around the interior of the hollowedcolumn structured supporting means 66 adjacent to the bimetallic disc12. The spacer 90 operates similarly to the spacer ring 30 of thethermal switch 10. The spacer 90 cooperates with the first land 68 toform an annular groove within which the peripheral edge portion 42 ofthe bimetallic disc 12 is captured. The spacer 90 is also provided withan aperture 92 sized to slidingly engage the drive pin 74. The aperture92 provides a track for guidance of the drive pin 74. When the drive pin74 is installed in the aperture 92 of the cylinder 90, the first end 84contacts the central mobile portion 49 of the bimetallic disc 12, andthe second end 86 is positioned for striking the sensitive portion 88 ofthe force indicator 72. According to one embodiment of the invention,the spacer 90 is formed of a thermally insulating material, such asglass or ceramic.

Spacing adjusting means 94 are optionally provided for adjusting theposition of the first end 84 of the intermediary drive pin 74 tocompensate for differences in the thickness of the bimetallic disc 12under test. For example, the spacing adjusting means 94 are embodied asshims provided between the cylindrical spacer 90 and the second end 86of the drive pin 74.

FIG. 8 illustrates the energy tester 60 of the invention embodiedwithout the intermediary drive pin 74. The supporting means 66 isembodied having the first annular step or land 68 around the interiorthereof and spaced above a base or lower edge 70 thereof. The land 68 issized to support the peripheral edge portion 42 of the bimetallic disc12 above the base 70. The force indicator 72 is spaced above thesupporting means 66 in sufficiently close proximity to the bimetallicdisc 12 that the snap force F is measured directly, without beingtransmitted through the intermediary drive pin 74. Accordingly, theforce indicator 72 is again suspended from the arm portion 73 of themeasuring device 64 overhanging the support structure 62 and thesupporting means 66. According to the embodiment illustrated in FIG. 8,the arm portion 73 spaces the force indicator 72 in sufficiently closeproximity to the bimetallic disc 12 that the snap force F generated bythe central mobile portion 49 is measured directly.

Spacing adjusting means 94 are again optionally provided to compensatefor differences in the thickness of the bimetallic disc 12. Theadjusting means 94 adjusts the relative spacing between the forceindicator 72 and the bimetallic disc 12 under test. For example, thespacing adjusting means 94 are embodied as shims provided between thebimetallic disc 12 under test and operational portion 88 of the forceindicator 72.

According to the method of the invention, the bimetallic discs 12 aresubjected to pressure testing that is performed in a prescribed manner,whereby the energy-tested bimetallic disc 12 of the invention is formed.

The bimetallic disc 12 is formed according to conventional methods byshaping a flat circular disc with a punch-and-die set to stretch thebimetallic material into the concave structure illustrated by the fullline 4 in FIG. 1. The bimetallic disc 12 is formed with a substantiallyplanar peripheral hoop portion 42 surrounding the central mobile portion49 that is stretched into a concave configuration. Examples of suchdish-shaped discs are illustrated in U.S. Pat. Nos. 2,717,936 and2,954,447, each of which is incorporated herein by reference in itsentirety. The formed bimetallic disc may be subsequently subjected to aheat treatment operation in order to achieve forceful snap-action ateach of the two predetermined set-point temperatures.

The dish-shaped bimetallic discs 12 are subjected to thermal testing,which determines the actuation or set-point temperature of eachindividual disc 12, and the discs 12 are categorized according to apredetermined methodology. For example, the tested discs 12 areseparated by material type into categories defined by low set-pointtemperature ranges of about 1 to 2 degrees Fahrenheit with predetermineddifferential temperatures.

According to the invention, the categorized bimetallic discs 12 arepresented to the force indicator 72 according to a prescribed manner fordetermining an amount of energy released by the thermally responsivesnap-action bimetallic disc 12. The method of the invention is embodiedas qualifying an energy released by transit of the mobile portion 49 ofthe bimetallic disc 12 from a first pre-snap side of the peripheral edgeportion 42 to a second opposite side of the peripheral edge portion 42during operation of a snap-action. The qualified bimetallic disc 12 issubsequently assembled into an operative relationship with a movableindicator portion of a thermal sensing device. For example, thequalified disc 12 is assembled with the thermal switch 10 in theposition described above for interacting with the intermediary strikerpin 46, whereby the qualified disc 12 moves the mobile contact 16 awayfrom the fixed contact 14 during actuation.

The released energy is qualified by presenting the disc 12 to a forcesensing device such as the force indicator 72. The disc 12 is presentedon the supporting means 66 while the central mobile portion 49 ispositioned on the first pre-snap side of the substantially immobileperipheral edge portion 42 opposite from the force indicator 72. Thedisc 12 is supported on its generally immobile peripheral edge portion42. The dish-shaped central mobile portion 49 is extended on the side ofthe edge portion 42 away from the force indicator 72. In other words,the central mobile portion 49 is withdrawn into the space 85 between thefirst annular land 68 and the base 70 of the hollowed column structuredsupporting means 66, the inner concave surface 48 of the central mobileportion 49 being thus spaced away from the force indicator 72. The disc12 is presented sufficiently closely to the operational portion 88 forceindicator 72 that the mobile portion 49 is positioned to forcefullyinteract with the operational sensing portion 88 of the force indicator72 during transit to the second post-snap side of the peripheral edgeportion 42 proximate to the force indicator 72. As described above, theforceful interaction with the operational sensing portion 88 of theforce indicator 72 is either direct or through an intermediary mechanismsuch as the drive pin 74.

The snap energy is released by thermally activating the bimetallic disc12 in the presence of the force indicator 72. The method thus includeschanging the temperature of the disc 12 to transit the central mobileportion 49 from the first pre-snap side to the second post-snap side ofthe peripheral edge portion 42 proximate to the force indicator 72. Themobile portion 49 of the disc 12 is moved into contact with theoperational portion 88 of the force indicator 72 during transit from thefirst pre-snap side to the second post-snap side of the peripheral edgeportion 42. To qualify according to the method of the invention, abimetallic disc 12 applies a minimum force F_(M) to the operationalportion 88 of the force indicator 72 during actuation. The operationalsensing portion 88 of the force indicator 72 thus senses the peak forceF_(P) generated by the transit of the central mobile portion 49 duringactuation.

The bimetallic disc 12 is thermally activated by either heating orcooling it through the set-point temperature using the thermal stage 78.Heating and cooling are under the control of the programmable controller80. According to one embodiment of the invention, the programmablecontroller 80 is used to control the heating and cooling rates ofthermal stage 78 so that the bimetallic disc 12 is heated or cooled attemperature rates as low as about 1 degree F. per minute or less. Thedisc 12 under test is thus qualified for long-term reliability indevices that must operate in conditions of very slow temperatureapplication rates without arcing. Optionally, the controller 80 is usedto control the thermal stage 78 at several different temperature ratesof change so that the disc 12 under test is thermally activated at aplurality of different controlled rates of temperature change. Snapforce F and set-point temperature data are taken at each actuation ofthe disc 12, and an energy-temperature rate relationship is determinedfor the disc 12 under test.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. Thus, it is to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described above.

What is claimed is:
 1. A method for determining a force generated by athermally responsive snap-action bimetallic disc during transit betweenfirst and second states, the method comprising: presenting the thermallyresponsive snap-action bimetallic disc to a force indicator on a supportstructure while a mobile portion of the disc is positioned on one sideof a substantially immobile edge portion opposite from the forceindicator, the disc being presented sufficiently closely to the forceindicator that the mobile portion is positioned to forcefully interactwith a sensing portion of the force indicator during transit to a secondside of the edge portion proximate to the force indicator; changing atemperature of the disc to transit the mobile portion into a position onthe second side of the edge portion proximate to the force indicator;and sensing with the sensing portion of the force indicator a peak forcegenerated by the transit of the mobile portion.
 2. The method of claim 1wherein changing the temperature of the disc includes changing atemperature of the support structure.
 3. The method of claim 1 whereinchanging the temperature of the disc includes changing the temperatureat a controlled rate.
 4. The method of claim 1 wherein the temperatureof the disc is below an actuation temperature of the disc prior tochanging.
 5. The method of claim 4 wherein changing the temperature ofthe disc includes increasing the temperature above the actuationtemperature.
 6. The method of claim 1 wherein presenting the disc to theforce indicator on a support structure includes simulating a portion ofa structure intended to support the disc during operation in atemperature sensing device.
 7. The method of claim 1 wherein changing atemperature of the disc to transit the mobile portion into a position onthe second side of the edge portion proximate to the force indicatorincludes generating a force with the mobile portion of the disc.
 8. Themethod of claim 7 wherein sensing a peak force generated by the transitof the mobile portion includes applying the force generated with themobile portion of the disc to the sensing portion of the forceindicator.
 9. A device for testing a force generated by transit of athermally responsive bimetallic disc between a first pre-snap state anda second post-snap state, the bimetallic disc being configured with asubstantially round immobile edge portion positioned peripherally to amobile center portion that extends on a first side of the edge portionwhen the disc is configured in the first pre-snap state and transitswith a snap-action in response to a predetermined set-point temperaturethrough the edge portion to extend on a second side of the edge portion,the testing device comprising: a columnar support structure having afirst annular support surface sized to support the edge portionpositioned peripherally to a mobile center portion of a thermallyresponsive bimetallic disc; a force indicator having a force sensingsurface positioned relative to the support structure to be forcefullyengaged by the mobile center portion of the thermally responsivebimetallic disc when the mobile center portion transits with asnap-action in response to a predetermined set-point temperature throughthe edge portion from a first side of the edge portion to extend on asecond side of the edge portion; and a thermal stage positioned relativeto the support structure to induce the predetermined set-pointtemperature in the thermally responsive bimetallic disc supported on thesupport structure.
 10. The testing device of claim 9 wherein the thermalstage is positioned adjacent and in close proximity to the supportstructure.
 11. The testing device of claim 10 wherein the forceindicator is suspended opposite the thermal stage.
 12. The testingdevice of claim 9 wherein the force indicator is a pressure-sensingtransducer of a type that is capable of sensing a peak force applied tothe force sensing surface.
 13. The testing device of claim 12 whereinthe pressure-sensing transducer is of a type that is capable ofdisplaying the value of the peak force in a useful manner.
 14. Thetesting device of claim 9, further comprising a drive member positionedintermediately between the annular support surface of the supportstructure and the force sensing surface of the force indicator fortransmitting a force generated when the mobile center portion of thebimetallic disc transits with a snap-action through the edge portionfrom the first side of the edge portion to extend on the second side ofthe edge portion.
 15. The testing device of claim 14 wherein thecolumnar support structure includes a second annular support surfacespaced away from the first annular support surface toward the forcesensing surface of the force indicator and sized having an interiordimension larger than the edge portion of the bimetallic disc; andfurther comprising a spacer that engages the second annular supportsurface and cooperates with the first annular support surface to form anannular groove within which the peripheral edge portion of thebimetallic disc is captured, the spacer including an aperture with whichthe drive member is slidingly engaged for motion between the annularsupport surface of the support structure and the force sensing surfaceof the force indicator.
 16. The testing device of claim 15 wherein thedrive member and the spacer are sized such that a first end portion ofthe drive member is positioned by the spacer to contact an inner concavesurface of the bimetallic disc when the disc is configured in a firstpre-snap state and installed into the annular groove, and a second endportion of the drive member is positioned by the spacer to engage theforce sensing surface of the force indicator when the disc is configuredin a second post-snap state.
 17. A testing device comprising: a forceindicator; a support structure that is spaced a predetermined distanceaway from the force indicator; a thermal stage positioned relative tothe support structure for changing a temperature of the supportstructure and wherein the support structure is spaced relative to theforce indicator such that, when a bimetallic actuator that is configuredin a pre-snap state is placed thereon and actuated to a post-snap state,the bimetallic actuator contacts the force indicator and a force sensingportion of the force indicator is positioned to sense peak forcegenerated by the bimetallic actuator by transiting between the pre-snapand post-snap state.
 18. The testing device of claim 17 wherein thesupport structure includes an annular land spaced above a base, the landbeing sized to support a peripheral edge portion of the bimetallicactuator above the base.
 19. The testing device of claim 17, furthercomprising an intermediary member suspended between the supportstructure and the force indicator, the intermediary member beingstructured to transmit a force generated by the bimetallic actuator to aforce sensing surface of the force indicator.
 20. The testing device ofclaim 17 wherein the force indicator is structured to display a value ofthe peak force.
 21. The testing device of claim 17 wherein the forceindicator is a conventional pressure-sensing transducer.